In recent years, remarkable development of a functional card such as an IC tag and an IC card has been achieved, and the functional card has come into use in an anti-theft tag, a tag for checking those coming in and out, a telephone card, a credit card, a prepaid card, a cash card, an ID card, a card key, a variety of membership cards, a bookstore gift card, a patient registration card, a commuter pass, and the like. Each of these antenna circuit constituent bodies for the functional cards is composed of; a base material made of a resin film such as a polypropylene (PP) film and a polyethylene terephthalate (PET) film; and an antenna circuit pattern layer made of metal foil of aluminum foil or copper foil, which is formed on a surface of the base material. The metal foil is bonded on one side or both sides of the base material with an adhesive interposed therebetween by employing a dry laminate method or the like, and thereafter, the metal foil is subjected to etching processing, thereby forming the antenna circuit pattern layer on the surface of the base material.
The conventional antenna circuit constituent body having the above-mentioned configuration, the conventional functional card equipped with the above-mentioned antenna circuit constituent body, and the conventional method for manufacturing the antenna circuit constituent body are disclosed in Japanese Patent Application Laid-Open Publication No. 2004-140587 (Patent Literature 1) and Japanese Patent Application Laid-Open Publication No. 2002-7990 (Patent Literature 2).
In addition, aiming at providing an antenna sheet having an excellent design characteristic which allows projections and depressions of an IC chip and an antenna circuit not to appear on a card surface and having excellent durability which allows functions thereof not to be impaired even upon exposure to a chemical agent and a high temperature, for example, Japanese Patent Application Laid-Open Publication No. 2005-100371 (Patent Literature 3) discloses an antenna sheet composed of a plastic film, an antenna circuit, and an IC chip, in which the plastic film has a void content of 5 through 50 volume percentage and which is a fine-void containing film stretched and orientated in at least one direction. It is described therein that it is preferable that a thickness of this plastic film is 25 through 500 μm; that it is more preferable that a lower limit of the thickness is 50 μm; and it is particularly preferable that the lower limit of the thickness is 75 μm.